In the mid 1990's it became obvious that the world-wide consumer appetite for bandwidth hungry applications would eventually mean a shift not only in the consumer electronics that deliver the “experience,” but also in the way that access networks would be deployed and used. At the time, while advances in data delivery over legacy copper networks (DSL for instance) and the implementation of hybrid-coaxial deployments seemed to suffice it was clear that in a short time both of these methods would have severe shortcomings to available end line customer applications. By the early years in this decade, the accelerated availability of high definition television programming, video-on-demand, VoIP, peer to peer gaming, IM, video uploading, etc, made the need for improved access immediate.
In 1998, ITU-T released the standard G.983.1, incorporated herein by reference, that was recommended by the Full Service Access Network (FSAN) group with the intent of working towards a truly broadband fiber access network. This initiative is generally known as the FTTH BPON, (“B” for broadband, and “PON” for passive optical network). One goal of this recommendation was making the delivery of data burdening applications, particularly high end video, as inexpensive as possible. At the physical layer, this means fully leveraging the almost unlimited bandwidth transmission capacity of a fiber waveguide, and for cost reasons sharing one central optical line terminal (OLT) over as many optical network units (ONU) as possible in a point to multipoint distribution configuration. A typical ratio is 16-64 ONU per OLT.
Implicit in the BPON recommendation is the ability to deliver the voice, data and video (e.g., the “triple play”) with specific designation to meet these requirements even at the physical layer. The type of information slated for transport in this specification can be broken into three types of services: broadcast (general and directed or narrowband), downstream, and return path services.
In a typical deployment there can be multiple hundreds up to thousands of PON in operation. Many of these PON are serviced by the same backbone transport system. “Downstream” is the specific information only particular to one ONU in a PON. Its delivery is managed by the OLT and dependent on the transport and networking specification use. Examples of downstream service include telephony, video on demand, and high speed data via ATM.
“Broadcast” is information that all ONUs of a particular OLT receive equivalently and exactly. Broadcast includes, for instance, nationally syndicated TV channels such as NBC, CBS, and ABC, or locally syndicated channels such as, for example, city council TV. “General broadcast” occurs when the same information is provided to all ONUs of many OLTs in a deployment (e.g., NBC, ABC, CBS, etc.) A “directed broadcast” occurs when all ONU's of a considerable subset of OLTs from of a deployment receive the same information (e.g., city council TV). Broadcast information and narrowcast information can be any form of data, voice or video. For example, in one instance broadcast information can be data such as QAM to a modem. Likewise, narrowcast information can be data such as QAM to a single modem or a subset of modems.
“Return path” is the upstream information that allows a closed loop information exchange system.
Generally, in a typical PON architecture there are four optical bands of operation: the 1270 nm to the 1350 nm band for the upstream, the coarse wave-division multiplexing (CWDM) band above the water peak up to 1480 nm for future upgrades, the 1480 nm to 1500 nm for the downstream, and the 1550-1560 nm range for downstream broadcast distribution. The hardware implemented is also particular with function and expectation. At a central office (CO) resides an OLT, which is an ATM based transceiver to transmit at 1490 nm and receive a 1310 nm signal generated by the ONU module. Also, at the CO is the placement of a 1550 nm transmission and the optical amplification necessary to transmit a broadband RF spectrum signal. The combination of the downstream signals and the drop of the upstream signal at the CO happen through a passive wide band filter. The input/output of this passive multiplexer is incident on one optical fiber and per the ITU specification can have a maximum logical reach of about 20 km for the BPON, with some distance variation for GPON and EPON configurations. Nearing the end of this PON distance there is a high count 1×N optical splitter, after which each fiber terminates at an ONU, typically a residence or business of some sort.
A typical ONU comprises an optical triplexer, which takes the input from the 1490 and 1550 nm upstream signals and separates them for independent reception, and takes the upstream 1310 signal and adds it to the PON fiber. Also comprising the ONU are the opto-to-electrical conversion properties of the receiver diode, amplification, and AGC circuit sets that prepare the signal for demodulation at the TV or set top box video receivers. The purpose of the video overlay (over the optical network (e.g., PON)) is to transmit a portion of the radio frequency spectrum (55 MHz to 1 GHz) to each ONU, a proven technology for high quality transmission of analog amplitude modulation and QAM. QAM modulation is Quadrature Amplitude Modulation, a symbol based modulation where amplitude and phase components exist according to baseband digital subsets. The QAM symbol capacity can differ, from 64 to 1024 symbol schemes, but most typically 256 symbol modulation is used. Currently deployable transmission capacity for the video overlay is quite large, up to 6.6 Gbps, which can support up to 1256 HD video channels, or 6594 SD video channels.
This RF modulation scheme and the leveraging of its transmission capacity have been perfected over the last two decades in HFC applications. HFC architectures have a fiber trunk that terminates at a node followed by a coaxial plant that distributes signal to the end uses. However, it can be advantageous to replace the coaxial distribution with a PON.
In RF transmission links, both the electronics and optics disrupt the input signal via various noise sources. The challenge for these types of links is that these impairments must all be managed or corrected to a certain extent for efficient interpretation by the end line user. The main noise sources to contend in these systems are: Relative Intensity Noise (RIN) from transmitter laser and laser to modulator interaction, and from optical amplifiers; intermodulation noise from transmitter, fiber, and fiber scattering; diode and electronic characteristics in the optical receiver module; and fiber non-linear interaction between multiple wavelengths. The relative intensity noise penalties degrade the RF signal to noise (CNR) parameter per channel over the whole operating band, the intermodulation noise creates harmonic beating effects (CSO from second orders, CTB from third orders) spread statistically throughout the operating band, and scattering phenomena appear due to the high launch powers necessary for cost effective delivery of signal, (SBS (stimulated Brillouin scattering) and SRS (stimulated Raman scattering) for multiple wavelengths interaction). All of these if unchecked reduce the necessary quality of service.
As a point of reference, in HFC, for optical fiber terminating at a node the specifications per channel are typically carrier to noise ratio CNR>52 dBc, composite second order (CSO)<−65 dBc, and composite triple beat (CTB)<−65 dBc, while for in FTTH for fiber terminating at an ONU the specifications per channel are typically CNR>46 dBc, CSO<−53 dBc, CTB<−53 dBc. For QAM transmission at an HFC node the specification desired is typically <E-9 symbol BER (bit error rate), while for a FTTH ONU only a <1E-6 symbol BER may be required.
With respect to the noise impediments, HFC systems are intermodulation limited. Thus all the technology development, network design, and cost reduction has gone mostly towards creating hardware that can mitigate intermodulation effectively. From the perspective of optical links, this means delivering to the coaxial plant very low levels of intermodulation distortions (e.g. −65 dBc), to be degraded rapidly through RF amplifiers to end delivery at customer site with some margin on typical standard (e.g. ˜53 dBc.) This limit has historically bound the evolution of optical networks in HFC. Specifically, this means that without due design provisions both at the board and systems level one would expect the CSO to go out of spec long before the CNR would.
FTTH systems, on the other hand, are more directly limited by factors of overall broadband noise sources which come from the interplay of composite laser modulation limits and in particular the shot noise coming from the optical to electrical conversion in the ONU receivers. These two points describe the maximum CNR per channel for FTTH systems. Practically, it is the case that for both technical reasons and cost scalability one wants to design FTTH architectures such that the broadband 1550 nm portions hits the receiver at the minimum value possible. For this case, FTTH systems are often referred to as shot noise limited. This limit however enables the use of multiple optical amplifiers in cascade, another distinction to HFC, where operating at optical input powers into the node higher than 0 dBm the RIN contribution from optical amplifiers can quickly dominate the CNR parameter.
One adverse, but necessary, point of comparison to HFC is that the optical link budget for PON recommendations is at or above 25 dB. It is known that while the physical limit of uncorrected sources is 7 dBm into fiber >25 km, which ultimately means that unlike for HFC links, that for FTTH the end of the optics link will be incident at a receiver at powers much lower than zero dBm, down to −8 dBm. This then leaves the receiver shot noise as the only dominant term to define the CNR for RF channels in the system, even to the point where other broadband noise terms, such as RIN from transmitters and amps are far secondary limiting factors. This benefit will become quickly apparent in the discussion of allowable optical amplifier cascades in FTTH.